For electronics, microelectronics and microelectromechanics, wafers composed of semiconductor material (semiconductor wafers) with extreme requirements to global and local flatness (nanotopology) are required as starting materials.
A wafer composed of semiconductor material is usually a silicon wafer, or a substrate having layer structures derived from silicon, such as, for example, silicon-germanium (SiGe), silicon carbide (SiC), or gallium nitride (GaN).
In accordance with the prior art, semiconductor wafers are produced in a multiplicity of successive process steps, wherein, in the first step, by way of example, a single crystal (rod, ingot or boule) composed of semiconductor material is pulled by means of the Czochralski method or a polycrystalline block composed of semiconductor material is cast, and, in a further step, the resulting circular-cylindrical or block-shaped workpiece composed of semiconductor material is separated into individual wafers by means of wire saws.
In this case, a distinction is made between single-cut wire saws and multiple wire saws, designated hereinafter as MW wire saws (MW=multiple wire). MW wire saws are used, in particular, when a workpiece, for example a rod composed of semiconductor material, is intended to be sawn into a multiplicity of wafers in one work step.
An MW wire saw is described in EP 990 498 A1, for example. In this case, a long sawing wire coated with bonded abrasive grain runs spirally over wire guide rollers and forms one or more wire webs.
In general, the wire web is formed by a multiplicity of parallel wire sections which are spanned between at least two wire guide rollers, wherein the wire guide rollers are mounted rotatably and at least one of them is driven.
The wire sections of the wire web can belong to a single, finite wire that is guided spirally around the roller system and is unwound from a supply spool (payoff spool) onto a receiving spool (pickup spool). The patent specification U.S. Pat. No. 4,655,191, by contrast, describes an MW wire saw wherein a multiplicity of finite wires are provided and each wire section of the wire web is assigned to one of said wires. EP 522 542 A1 describes an MW wire saw wherein a multiplicity of continuous wire loops run around the roller system.
The longitudinal axes of the wire guide rollers are oriented perpendicularly to the sawing wire in the wire web.
The wire guide rollers generally consist of a core composed of metal, which is usually enclosed longitudinally with a jacket, for example composed of polyurethane. The jacket has a multiplicity of grooves that serve for guiding the sawing wire which establishes the wire web of the wire saw. A wire guide roller optimized with regard to surface coating and groove geometry is describes in DE 10 2007 019 566 A1.
The production of wafers composed of semiconductor material makes particularly stringent requirements on the precision of the slicing process. The sawn wafers are intended to have plane-parallel side surfaces which are as flat as possible. In order that the sawn wafers can arise with such a geometrical characteristic, an axial relative movement between the workpiece and the wire sections of the saw web, that is to say a relative movement parallel to the central axis of the workpiece, must be avoided during the sawing process.
For this purpose, it is important that the multiplicity of grooves in the jacket of the wire guide roller run exactly parallel and the grooves and the sawing wire lie in one line (alignment) and the position or the cut-in angle relative to the workpiece does not change. If such a change (alignment error) takes place, wafers having a curved cross section (warp) arise.
As a cause of the change in the position or the cutting angle of the wire sections of the saw web, that is to say the relative movement of the wire sections parallel to the central axis of the workpiece, US 2010/0089377 A1 mentions temperature changes and an associated thermal expansion or thermal contraction of the workpiece and of the wire guide rollers.
In the course of the sawing process lasting a number of hours, heat arises both as a result of the sawing process itself and as a result of the sawing wires running around the wire guide rollers, said heat being transferred to the workpiece to be sawn and also to the wire guide rollers.
According to DE 10 2011 005 949 A1, thermal expansion of a single crystal composed of silicon having a diameter of 300 mm is approximately 25 μm if the single crystal is heated by 30° C. during wire sawing. Thermal expansion can be avoided by the single crystal being cooled during sawing.
In accordance with the prior art, thermal expansion or thermal contraction (thermally induced change in length) of the workpiece is minimized for example by a cooling medium being applied to the workpiece during wire sawing. However, the effect of this cooling on the wire guide rollers usually is insufficient for maintaining strictly stable thermal conditions.
The heat that arises as a result of the wire sawing process can also lead to a thermal expansion of the wire guide rollers spanning the wire web, as a result of which an alignment error can occur, that is to say that the sawing wire no longer cuts into the workpiece at the angle applicable at the beginning of the sawing process. Thermal expansion of the wire guide rollers spanning the wire web can thus lead to an impaired wafer geometry in the sliced semiconductor wafers.
There are various approaches in the prior art for minimizing or avoiding the alignment error caused by thermal expansion of the wire guide roller and/or of the jacket enclosing the core of the wire guide roller.
The document DE 11 2008 003 339 T5 describes a method wherein the temperature of the slurry fed to the wire web is increased continuously from the beginning to the end of the slicing process. The method is based on the observation that with increasing length of engagement and with increasing progress of the slicing process, the rod becomes hotter and hotter and the position of the slicing gaps relative to the other components, in particular the wire guide rollers, thus shifts. This leads to wafers having front and rear sides substantially curved relative to the intended plane of cutting. The continuous increase in the temperature of the wire and of the wire guide rollers by means of hotter and hotter slurry over the cut ideally brings about a thermal expansion of the wire guide rollers synchronously with and to the same extent as the rod, such that wafers having substantially flat front and rear sides are obtained.
The German patent application DE 10 2011 005 949 A1 describes cooling the wire guide rollers and the fixed bearings thereof independently of one another.
DE 102 20 640 A1 and DE 693 04 212 T2 describe methods for monitoring and, if appropriate, correcting the alignment of the sawing wire with respect to the grooves in the jacket of the wire guide rollers. By way of example, DE 693 04 212 T2 describes a positional control of the wire guides which constantly measures the position of the wires by means of a detection system, wherein the detection system cooperates with a compensation device in order to keep the position of the wire guides unchanged relative to the workpiece to be sawn. However, the detection system can be influenced both by the grinding medium and by the abraded material arising as a result of the sawing process to the effect that measurement errors occur.
The German patent application DE 195 10 625 A1 describes the use of wire guide rollers composed of a glass-ceramic material that tends toward a very low thermal expansion, which are additionally mounted between a fixed bearing and a movable bearing in order to compensate for a thermal expansion of the wire guide roller. Glass-ceramic materials have proved to be unsuitable in practice with the use of a grinding medium containing abrasives, since the sawing wires cut into the workpiece after a relatively short time.
A further method for avoiding thermal expansion of the wire guide rollers in a wire saw is to set a constant temperature in the core of the wire guide roller by means of a corresponding temperature-regulating device.
The patent specification DE 695 11 635 T2 describes a wire guide roller having a core subdivided into two inner regions, a coolant circulating in said core. A temperature gradient within the core is intended to be avoided by means of the two independent chambers.
In addition to avoiding thermally induced expansion of the core of the wire guide roller, avoiding or restricting thermal change in length of the jacket longitudinally enclosing the core of the wire guide roller is also crucial since the jacket with its grooved profile directly influences the alignment of the wire sections relative to the workpiece. Thermally induced change in length of the jacket of the wire guide roller is dependent, in particular, on the coefficient of linear expansion of the jacket material, on the thickness of the jacket and on the quantity of heat arising during the sawing process.
The jacket is typically fixed on the core of the wire guide rollers in such a way that it can expand or contract axially at both ends in an unimpeded manner in the event of a temperature change. DE 10 2011 005 949 A1 describes a method for slicing wafers from a workpiece by means of a wire saw, wherein the fixed bearing of the wire guide rollers and the wire guide roller are cooled independently of one another in order to reduce or completely prevent an axial relative movement of the workpiece and of the wire sections of the wire web that are guided by the wire guide rollers during the sawing process, that is to say that an equidirectional change in length of the coating and of the fixed bearing is effected in reaction to a change in length of the workpiece during the sawing process.
Furthermore, the application DE 10 2011 005 949 A1 describes that the change in length of the jacket can be restricted within certain limits by the coating being clamped onto the underlying core of the wire guide roller, for example by clamping rings arranged at both ends of the coating. The clamping rings fix the jacket on the core of the wire guide roller and restrict a change in length of the jacket that is caused by a temperature change.
However, DE 10 2011 005 949 A1 does not teach a method of utilizing the different expansion of the core material and the jacket surrounding the core of the wire guide rollers spanning the wire web in a targeted manner for improving the geometry and the waviness of the wafers sliced from a workpiece.